text:
Boundary Markers of European Airspace
As in previous question I am trying to transpose a geospatial data format from within a EUROCONTROL application into something I understand, like decimal lat-lon.
An answer to my previous question Reference Map of EUROCONTROL charging zones got me a long way forward. I now understand that the data references this map:

This is the result of plotting the data in the file directly onto the screen. In the case of EG (UK airspace) I've added little circles to show the coordinates.

What I now need to do is to take some of the points within my file and have the actual coordinates in lat-lon (or indeed any commonly used format).
It seems to me that the boundaries of the map might be set by waypoints (or perhaps vice versa). I grabbed this image from aeroresource.co.uk and annotated it to illustrate my theory.
My assumption
It looks to me like the boundary of north east boundary of EG matches with the line of waypoints indicated by the annotated green line. So the north boundary starts with RATSU to the east and GUNPA to the west.
The easternmost boundary runs from RATSU to the north to NIBOG to the south.

And finally, to my questions
Is that assumption correct? Is the boundary of the various airspace zones delimited by waypoints?
If yes - is there an authoritative source of that information?
If no - is there an authoritative source of whatever points are being used to draw the bounds of the airspace?

If your question is "Is european airspace delimited by markers RATSU, GUNPA, and NIBOG", the title should reflect it. See the help center for more information

Rather than update the title, I've updated the question.

You should consider how charges are calculated in areas with delegated airspace. For example, Denmark controls a bit of German and Scottish airspace. I don't know if flights through those areas pay to Denmark or to the other countries. Those boundries do not match the map you have.

The boundaries as I drew them are lifted directly from the EUROCONTROL application designed for the specific task of calculating nav charges, so I'm confident I've got the correct data for my requirements.

@BenMayo you should also update the title as this is a Q&A website and more importantly it helps navigate through questions without opening each question.

My aim here is to find the information on boundary markers of European Airspace. The question is titled accordingly. A map of the boundaries is drawn from a geospatial dataset. I'm looking for that dataset or failing that, to find a correlation with another - such as a waypoint. Just anything where I can say 'the north east corner of EG is xxx yyy' and from there reverse engineer the other coordinates.

No, your assumption is not correct. Airspace boundaries are not defined by waypoints, but by WGS84 lat/lon pairs, which you can probably just use directly without a lot of manual work.
You need to consult the national Aeronautical Information Publication (AIP) for the country in question to get the coordinates for their national airspace, called a Flight Information Region (FIR). Specifically, look in AIP section ENR 2.1 (all AIPs worldwide are structed in the same way). Here is an example for the UK: https://www.aurora.nats.co.uk/htmlAIP/Publications/2020-03-26-AIRAC/html/eAIP/EG-ENR-2.1-en-GB.html#ENR-2.1
I recall having downloaded a global file with FIR coordinates for the entire world somewhere off the web, although I don't remember exactly where. It may well have been published by the ICAO. You might be able to locate it if you look around a bit.

However, many of those lat/lon pairs may end up as waypoints for convenience of handoffs between ATSUs, and because routing policies and services may be different on each side.

Sure, a lot of waypoints are located along the borders of FIRs. Not so commonly in the corners though, and that's what OP needs

Success - I took the London FIR points, converted them to decimal lat-lon and plotted them on a map. The numbers being used within the application are 60 x the decimal lat-lon. Which makes sense.

The boundaries of the European airspace are not defined by waypoints. There are however many waypoints on the boundaries. This is mainly for practical purposes related to air traffic control. These waypoints are used as clearance limits and transfer points for coordination between adjacent Flight Information Regions.
I think the additional points you found on the east-west boundary in the north have a different purpose. The boundary is along a circle of latitude. Normally, when you would just use the two endpoints of that line segment, you would create a great circle which is not along the circle of latitude. By inserting addional boundary points, the deviation between the great circle line segments and the circle of latitude (a specific case of a rhumb line) is limited.

souce


text:
Tapered Wing Providing low Coefficient of Lift
Hi I am currently analyzing different wing designs using ANSYS Fluent.
One of my designs has a straight leading edge but a swept trailing edge thus it is tapered with wing root chord at 50mm and wing tip chord at length 200mm. Wing span is 250mm. (See Picture)
The airfoil shape used is the S1223 which has a CL max around 2.3. Therefore using the Lift formula which (i know is approximate), I calculate the wing can produce 58.5N of lift at 36.452m/s.
However using ANSYS Fluent I find the wing can only produce a maximum of 39N of lift which is a CL of 1.5.
I am fairly certain the simulations I'm running are accurate but need advice on what could be causing this difference.
Assuming the results I've obtained are correct. What is the reason for the large difference in coefficient of lift ?
Let me know if I need to provide any additional information!
Any help greatly appreciated!
EDIT : Forgot to add that the simulation used assumes the wind tunnel boundary is at wing tip and wing root. Thus the wing span takes up the full wind tunnel.

the image seems to show the wing inverted -- are you measuring lift in the correct direction for the camber?

Not sure what you mean by "wind tunnel boundary". You should set the plane corresponding to wing root to be symmetry boundary condition. There should be no boundary on the tip except the normal wall boundary for near field. The far field should be velocity inlet and the trefftz plane should be pressure outlet.

I'm actually measuring the downforce that the wing produces so yes I am measuring in the correct direction @ZeissIkon Thanks for your response.

@JZYL I had set the boundaries to be at the tip and root for the time being, just to determine coefficient of lift neglecting any wing tip vortices. Both were set to be symmetry boundary condition. I have just corrected this according to your advice however there is no change in the lift value I am getting. However there is a small increase in drag. Thanks for your response

what is your question?

Assuming the results I've obtained are correct. What is the reason for the large difference in coefficient of lift ? @ManuH

At a high CL (or high angle of attack), for a viscous simulation, you will have lots of flow separation, which will reduce your lift. Since the airfoil is inverted, the suction side is the lower surface, and that's where you should see an adverse pressure gradient leading to reversed flow.

@JamesGreen you should put your last comment in the question so that it is clearer at the first reading.

@afcdesign ok great thank you makes sense, I will have to read up a bit more. I've added a picture of my Pressure contour at a high AoA, does this show an adverse pressure gradient ??

@ManuH I have done now thanks for pointing that out

What angle of attack do you use? Do you vary it enough to cover the maximum lift case? Would be interesting to see the lift over AoA plot.

@PeterKämpf I varied AoA up until it reached stall point. Have added graphs of 2D analysis and the 3D tapered analysis. It is worth noting that rectangular wings of the same shaped airfoil closely follow the 2D trend. I'm just looking for a conclusive explanation as to why the tapered wing does not produce a closer CL max.

@JamesGreen you will see an adverse pressure gradient after the suction peak on the suction side of the airfoil. The question is what is the extent of flow separation at high angles of attack. You could see it more clearly if you plot the surface skin friction or velocity profile.

@James Green some similarities in your design and data to sails.  You may wish to try various camber ratios, say from 1/20 to 1/7.  Love the pressure contours for ground effect study.

By tapering the wing you have in effect a 3D case even if the wing is sitting between wind tunnel walls. Lift varies along span almost as if the wing would be in free flight with twice the span and the long chord at the root. Since tip chord is only a quarter of root chord, that full wing would have 0.075 m² area with 0.5 m span, giving it an aspect ratio of 3.33. Below is a figure from Sighard Hoerner's book Fluid Dynamic Lift (Chapter 3, Figure 4) which shows how the local lift coefficient varies over span for wings of different taper ratio. Your wing in free flight would be ¾ between the second and third from the left.

A 2D case is only possible with constant chord as all of the wing would produce the same circulation (lift times chord). With the varying chord along span only the shorter end will reach the maximum lift coefficient of 2.3 locally, but at a lower angle of attack. You still get more lift with your wind tunnel walls than in free flight, but with a taper ratio of 0.25 not much of the 2D case is maintained.
The higher circulation of the long chord end produces extra circulation on the short chord end and lets it stall at a lower angle of attack when the root has only reached half of its full potential (see figure above where center lift coefficient is only half of the tip lift coefficient). The reduction in maximum angle of attack from -13° for the 2D case to -10° also shows how much potential lift is lost.

thanks very much for your time and answer. Greatly appreciated. So as I understand only the shorter chord length reaches maximum lift coefficient? But the long chord end has higher circulation ? I though lift was directly proportional to circulation. Or are you saying that this is true however due to the large difference in circulation there is surplus circulation on the shorter chord length leading to loss of lift ?

@JamesGreen Yes, only the shorter end reaches maximum lift and does so at a lower AoA. See the first kink in the lift curve slope: I expect that it is caused by separation onset on the short end. While the lift on the long end increases with increasing AoA, the short end stalls already. Did you try washout (different incidence between root and tip)? Regarding circulation: Yes, it is proportional to lift and high root circulation causes an AoA increase at the tip which lets it stall prematurely.

source: Stack Exchange

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sort: votes


text:
What is DME Unlock?
Reading through a NOTAM I found something I don't understand,
A0311/17 NOTAMR A2635/16
Q) VTBB/QIDXX/I/NBO/A/000/999/1342N10046E025
A) VTBS  B) 1702080225 C) 1705081100
E) ILS DME I-SES CH38X RWY 01R UNUSABLE BEYOND 15DEG
LEFT OF LOC COURSE DUE TO DME UNLOCK

What is DME Unlock? How does it affect the flight?

Could you post the NOTAM in full (or at least provide the location identifier)?

@J.Hougaard added. Now it is very long. The part I am interested in is the last line.

I think it just means a situation in which the DME is "no longer locked on".  Sounds like the DME may not be seen properly from those areas mentioned in the NOTAM.
From the Naval Instrument Navigation
Workbook

TACAN Characteristics
1.Bearing/Distance Unlock. TACAN bearing and distance signals are subject to line-of-sight restrictions because of their utilization of
  UHF frequencies. Because of the transmission/reception principles,
  unlock (indicated by rotating of bearing pointer and/or range
  indicator) will occur if these signals are obstructed.

...

Aircraft receiver memory circuits prevent unlock for short
  periods (approximately 10 seconds for DME and 2 seconds for azimuth.) But beyond this unlock
  occurs and will persist until the obstruction is removed and search cycles are completed. Unlock
  may occur during maneuvers, such as procedure turns, which cause the aircraft antenna to be
  obstructed for longer than 2 to 10 seconds

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Are pets/animals in the cargo hold monitored inflight for any sudden medical condition?
My question is more from a long haul flight POV. Does the crew do a mid-flight check etc. What if any animal/pet have a sudden cardiac arrest or some emergency medical situation, will the crew or pilots be alerted for the same?

I don't believe that the crew (flight-deck or cabin) have any access to the cargo hold on most commercial airliners, so if an animal in the hold gets sick, it gets sick. I'm sure that for special, high value animals, say transporting an elephant, that is different, but they usually don't go below decks and travel on an extra baggage charge for the human upstairs. I may sound cold, but I love my dogs as much as a normal person should - we took our 2 dogs JKF-TLV when we moved there. I would have been very sad had something happened to them, but they are just pets...

As for my personal experience no since my dog was inside of a cage and there was no way to check what was going on inside.

@FreeMan That sounds like an answer to me.

Thanks, @DanHulme, I don't think anyone's ever said that to me here before! :)  Unfortunately, I started with "I believe" for a reason - I'm not really sure about it, thus the coment & not an answer...

@FreeMan I guessed as much, but that's not what comments are for. If you post an answer, people can vote to indicate their agreement. If it turns out you're completely wrong and it ends up negatively scored, you can always delete the answer - that also wipes out the reputation change.

To the best of my knowledge, no modern (i.e. currently used in regular commercial service in statistically significant numbers) commercial passenger aircraft have access from the main passenger cabin to the cargo hold, so there is no way for the cabin/flight-deck crew to check on the critters below, even if they wanted to. My knowledge, however, is incomplete, as was pointed out in the comments, the 747-100 & -200 (and likely the -400) do have access from the passenger cabin to the forward cargo hold. The flight crew were notified when animals were stowed there, however, a long-serving 747 pilot indicated having never checked in on said animals.
Large and/or high-value animals such as elephants, giraffes, etc. are shipped as cargo via specialist transport companies where arrangements to have them tended in flight are common and accounted for (hat tip to Simon). I doubt you'll see someone pulling up to the departures gate at LGA with an elephant in a crate and wheel him up to the curb-side check-in gate.
As a personal anecdote: When I flew from JFK/KJFK to TLV/LLBG in the mid-80s we took our 2 dogs since we were moving there for a period. We got to walk the dogs on the tarmac just before they were loaded in their crates and put in the hold (747, probably -200) at JFK, but there was no mention of anyone checking on them mid-flight.
Fun fact (for me): We also got to walk the dogs at TLV just prior to our return flight, and that walk ended up taking about an hour as we got to watch them jack up a wheel on a 747 to change a flat tire pre-flight. (Yes, security was much different then. Even in Israel at the height of their occupation of southern Lebanon.)

Rare, large and high-value animals are normally freighted by specialist cargo carriers and their carers, vets etc ride along with them.

@Simon - that was my point. Probably wasn't well made.

Flying Tigers used to transport a lot of animals, including livestock. I was told by fellow employees that after FedEx bought FT the 747s we retained from their fleet still had a foul odor in the cargo compartment.

The forward cargo hold of 747-100/200 aircraft is accessible inflight. My guess is that is also true for 747-400 aircraft, but I do not personally know that. However, the access is for maintenance and emergency purposes. You lift a patch of carpet in the aisle just aft of the nose on the main deck, open a hatch, and descend a built-in ladder down into the avionics bay. If you go forward, you'll be at the nose gear emergency extension station. Go aft and take down a canvas barrier, and you're in the forward cargo hold.

Well... a 747 isn't modern, it's almost as old as I am! Wait, it's still in use currently and therefore meets my self-defined criteria of "modern"? Crud. Humbly bows to our resident 747 expert, @Terry

@Terry - I believe you flew many pax flights as well as cargo in those wonderful old -100s & -200s. Were there ever any times when live animals in the cargo hold were checked on? You might be our most authoritative expert on the situation.

Good answer and comments. Though most answers are about access to the cargo hold, which I deduce, is not available in most flights. What about the monitoring part, like a CCTV camera, or some other instruments to measure the sound, if any animal is crying in pain, or to monitor the temperature in the cargo hold etc.

That seems to be a valid additional question, @Firee, but what would they do - turn around or divert 1/2 way across the Atlantic? Seriously, I loved my pups when I was a kid (and love our dog now), but I wouldn't want or expect them to inconvenience an entire plane full of people who have places to go and things to do because my dog was freaking out or getting sick down in the hold. If it was a cat, I'd be looking for the eject button, but that's just a personal prejudice...

@FreeMan As I remember, we were typically notified (actually the flight engineer was notified) that there were animals on board. I'm certain, though, that we never checked on them in flight in the forward cargo hold.

The exception to the rule that access to the cargo hold is not available is FedEx (and other freight haulers), who routinely transport zoo animals and other research animals.
In passenger service, generally most cargo holds are not readily accessible.  There are some exceptions.
The inaccessibility of the cargo hold is exploited when US agencies transfer classified materials. Couriers will place a container into the hold just prior to the doors closing, and then wait for the aircraft to taxi from the ramp. At the destination, a courier will meet the aircraft at the ramp, and when the cargo doors are opened, the courier extracts the container. Other countries and NATO have similar procedures.

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text:
Which commercial jet aircraft have the option to install ACT (Additional Center Tank)?
Which commercial jet aircraft have the option to install ACT (Additional Center Tank)? I think A321 has this option but I am not sure which other commercial jets have this option or this can be done with almost any plane?

'Plane' covers a LOT of aircraft, ranging from home built experimental craft up to military experimental craft. Surely it's clear that not ALL aircraft will have this option and ability?

modified it...   i was mainly interested for commercial jets..  i know many small business aircrafts have this option.

The boeing 737 does. They add a fuel tank in the center of the fuselage and when there is a fuel tank is added it is called an ER(extended range). Boeing 737-900ER is the most common aircraft I have heard of with an ACT

Is it a fixed sized ACT or can have different versions? Any idea on that?

Since this question has two downvotes, I am voting to close this question.

In terms of a non permanent installation almost any aircraft can be plumbed to accept a Ferry Tank(s) this is typically done for smaller aircraft to transport them over open water distances they can not otherwise cross. As far as I know this is not really done in commercial operations but you can add tanks.
This article seems to confirm the center tank on the A321.
If you are asking about a tank that can be added and removed quickly I do not know of any but from a regulatory standpoint doing so would require separate weight and balances to be prepared for both cases.


text:
What were to happen if deicing fluid went in to the jet-engine?
What were to happen if deicing fluid went in to an engine. I have heard that deicing fluid is somewhat flammable. What were to happen if it went in to the engine? Would it combust like fuel?

I edit the question to refer to deicing fluids use on wings and other structure. If this is not what you mean feel free to roll back.

It would be de-iced, of course.

Alcohols are typically very effective de-icers (consider that ordinary vodka won't freeze until -17*C, and ethylene glycol, a "diol" or "double alcohol" with two hydroxyl groups, is used as an antifreeze in automotive coolant), but virtually all of them are flammable in some concentration, so it could affect the engine, but unless the engine drew straight deicer through the fuel line it would be unlikely to cause a major problem.
First, it would depend on exactly which deicing compound is being used; the most common are methanol, ethylene glycol, propylene glycol and glycerin. These all have different flash points and different heats of combustion (the amount of energy released by burning them) which will differ more or less dramatically from those of 100LL or Jet A. The closer a compound is in its combustion characteristics to that of the actual fuel, the more it will act like the fuel and thus the fewer problems you'll have; ideally, a compound that is indistinguishable from the aircraft's fuel will simply result in a richer mixture if it's ingested through the air intake (which can be easily corrected with the mixture knob), while if it's fed through the fuel line you'll see no difference at all in the short term (in the long term the de-icer might cause more buildup of combustion byproducts inside the engine requiring cleaning).
If there is a difference in combustion characteristics, you'll see a difference in engine performance relative to the amount being ingested by the engine. A lower flash point for the de-icer means it will ignite before the fuel does and can cause misfires in the engine which is bad (the timescale for exactly how bad depends on the severity and timing of the misfires and the type of engine). A higher flashpoint will cause the engine to lose power, as the de-icer won't burn as efficiently in the combustion chamber, and if ingested through the air intake, will displace oxygen.
De-icer mixed with water is probably the worst case scenario, as under no circumstances will fuel, de-icer and water all sit in a homogeneous solution. Again it depends on how much the engine gets, but the addition of water will act to quench combustion and can cause an engine failure that you're unlikely to recover from in the air (once water gets into an engine it's very difficult to get it back out).
With a jet engine, most of these considerations are reduced by the simple fact that as much as 80% of the air entering the nacelle never enters the actual combustion chamber. So, ground crews at major airports can be much more liberal with de-icer around engine intakes (and should be, as the pressure differentials inherent in and around a jet intake can increase ice crystal formation). Further, commercial jet engines have to be rated to withstand ridiculous amounts of ingested materials of all kinds, including water, birds, dust/sand, etc. A little de-icer is probably the friendliest foreign substance to be introduced into a passenger jet's turbofans during its service life.
Finally, a simple search for "anti-ice" turns up several variants of aviation and automotive fuel with de-icing components mixed in; it's especially important for automotive fuel as most of these fuels contain ethanol which is hygroscopic (it will pull water out of the air) causing ethanol-water ice crystals to form in the fuel lines. AvGas is an older formulation that still uses tetraethyllead and no ethanol, but water contamination is still possible and so a fuel stabilizer (which also typically acts as a deicer) is a common addition to winter fuel tanks. So, aircraft manufacturers know that de-icer will be fed into the engines in some way or other, and they design those engines to be tolerant of it.

The flame in turbine is continuous and hot enough that anything will burn in it. The flash point will have minimal effect on the result. Whatever gets digested and is combustible will burn.

Too much fuel is a big problem for turbine. It increases pressure in the hot section, which can cause a compressor stall, which in turn causes shortage of air and flame out. This is more likely at full power when the engine is running close to the limit of what the compressor can do.

Turbines (and diesels for that matter) don't have mixture knob. They only have power lever which simply controls the amount of fuel injected. Before advent of FADECs, it was important to advance the power levers smoothly, otherwise you could put in too much fuel and flame the engine out, with or without a bang (either compressor stall or the too much fuel would simply not have enough air to burn with and the temperature would drop below the flash point).

You seem to talk about what would happen if the deicing fluid got into fuel line of a spark-ignited reciprocating engine, while the question clearly says jet engine and seems to assume ingestion via air intake.

Deicing fluid is inhaled by jet engines nearly every time the truck is called out. The question is how much deice can the engines tolerate?
Turbine engines can accept a rather staggering amount of water, to the point that testing them becomes an exercise in water handling instead of engine handling. The fire department can empty their trucks into an idling engine and not affect it.
Deice does contain various flammable liquids, but the overall mixture is not flammable - rather dangerous although fire would be a quick way to clear the ice.
So, unless you find a way to empty the truck into the engine in less than a few seconds it's simply not an issue.

It would not explode it would combust.    It would combust at a different rate from jet fuel but unless it was at large quantities (like in the fuel not just sucked off the plane) it should not cause any problems.
Combustion is oxidation and is limited by mass transfer delivering the oxidizing element (oxygen).
In an explosion all the components are in molecular close proximity and the rate is only limited by kinetics.   A bomb.
Alcohol is kind of in between as oxidation liberates oxygen but it is still mass transfer limited.


text:
What does RTF mean?
There is a sign in Tempelhof airport in Berlin. What does the abbreviation RTF mean? I tried to google but found nothing.

Read the f... Oh, wait. That's something else.

You're missing an M though, so unless internet tweens have gotten even lazier, which wouldn't surprise me! would suspect this to mean RadioTelephone

This is no excuse not to read the effing instructions.

@DavidRicherby Looks the same to me! (RTFI)

"Read the fantastic" Instructions?

@coteyr It's actually quite common to just use "RTF" if you're supposed to be reading something other than the manual. On Stack Exchange, one quite often wants to tell somebody to "RTF question" (though one doesn't because that isn't nice). So "RTF instructions" is idiomatic IM(F?)O.

Honestly, it made me more curious about the "follow me" instructions.

@coteyr https://c1.staticflickr.com/8/7240/7084307087_992d1d3ae5_b.jpg

"RTF Instructions" probably means "Radiotelephone Instructions"
As suggested here, it should not be confused (as I initially did) with the "RTF" of internet fame (from RTFM, for those that are still wondering after reading the translation below), meaning something completely different:

Ophelia und ich, die beide mehr Ahnung vom Internet als von der Luftfahrt haben, konnte hier drüber lachen: Hold for Follow Me or read the f[-]ing instructions. Tatsächlich wird es wohl bedeuten, dass der Pilot entweder das Follow Me-Fahrzeug abwarten soll oder den Anweisungen des Towers folgen, die über Sprechfunk (Radiotelephone = RTF) vermittelt werden.

translated

Ophelia and I, both of whom are more familiar with the Internet than aviation, could laugh over it: Hold for Follow Me or read the f[-]ing instructions. In fact, it will probably mean that the pilot should either wait for the Follow Me vehicle or follow the instructions of the tower, which are transmitted via radio (Radiotelephone = RTF).

This is also confirmed by this source where "RadioTelephone (RTF)" is listed in several occasions.

Context in terms of location would be helpful, but it could mean "Return To Field."
For example, if this sign were placed in an overrun area or access road adjacent to a taxiway it is possible an aircraft that lands long or gets confused taxiing during low visibility might end up there.
In such a case a ground controller probably wouldn't want the airplane to try to figure out how to get back to a controlled movement area on their own, but would want to send a follow-me truck or give them clear instructions how to return to the field.

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text:
Can experts identify the model using this part (FOD prevention purposes)?
We found this part of an aircraft recently and, in order to improve the safety levels at our airport, we would like to know what possible aircraft model (or models) are compatible with this cover so we can locate it.

Hey, where did the real answer go?!?

This is a Military Standard fuel cap, part number MS29525-1.
Here is the DOD page for the products: MS29525-1. It is most likely from a military aircraft or some kind of private ex-military warbird.
There really isn't much to say on it, other than it is used by the Air Force and Navy, as well as some British aircraft. It is a universal part used on many different types. It is a serialized part, you could try calling a local airbase and see if you can get records for it, but they will probably not care enough to track it down.
By the way, whoever is missing it, if a private party, paid a dear price for losing it. The cost of these caps in the private market range from \$600 to \$800.

$600!! Is it made from gold?!!

@curious_cat: The certification document is the expensive bit. It's just ink and paper but it cost the company that made that part a lot of money (and still cost them if they keep manufacturing the part). The cost to actually manufacture it is probably around $50

@mins: Certification, no, tracking yes. For every part, the origin can be traced down to the moment of manufacturing, and often the details of manufacturing as well. I recall an investigation where a defect was traced back to the batch of metal used, which turned out to be a mistake at a foundry. Real experts can probably tell you which parts are important enough that they do get certified individually. (I suspect engines yes, pax seats no)


text:
What are the elements of an approach briefing?
I've heard that before landing an airliner, the pilots perform a mini-briefing to agree on what they will do and what they can do if something goes wrong during the approach and landing.

Is this true?
What is it called? (my best guess would be "approach briefing")
What elements does the briefing contain?

Just to let you know, I edited my answer to cover "what if something goes wrong?"

Yes, it's true.  Usually called approach or arrival briefing
Details will vary according to aircraft and operator but a typical list is NATS:

NOTAMs1
ATIS2 notes, chart briefing point by point, date, frequencies, courses, minimums and missed approach procedures.
Terrain, minimum safety altitude, which route
Special notes for arrival, taxiing, likely gate for arrival?

I noticed that you specifically asked about something going wrong.  The following assumes a serviceable aircraft.
According to operator, if the aircraft is not on a stabilised approach by 1,000 feet, a go around will be performed and the missed approach procedure (MAP) followed.  The MAP is printed on the approach chart.  The initial action is:

to get the aircraft safely climbing away (aviate)
to the altitude and heading defined in the MAP (navigate)
then call ATC to let them know (communicate).
Aviate, navigate, communicate, always in that order.

A stabilised approach might differ between operators but is essentially.

The flight path is correct with only minor changes in pitch and
  heading needed to stay on it.
No more than 20 kts above the approach reference speed briefed as above.
Correctly configured for landing (flaps, slats, gear, spoilers armed, autobrake set etc)
Checklists completed
Rate of descent <= 1,000 fpm
Power setting is appropriate
There may be other special checks for some types of approaches which must have been briefed as above).

The MAP might be something like:
Maintain runway heading and climb to 1500' until reaching 4 DME.  Turn right heading 250 degrees and climb to 5000' to join the hold at DISNEY
The MAP is unique for each approach so must be briefed in advance.

1
Notice to Airmen: Warn of recent changes, temporary restrictions, equipment malfunctions etc.
2
Automated Terminal Information Service: A continuous broadcast of airfield information such as weather, special precautions, runways in use etc.

how about identifying navaids, setting comm freqs

@rbp I think I got those - chart briefing point by point and frequencies?

i get that, but the way its phrased it sounds like you're talking about it rather than actually doing it

@rbp Doesn't the checklist follow the briefing?


text:
Does the pressure change inside a plane when it is opened for paratroopers?
Does the pressure change inside a plane when it is opened for paratroopers?

It depends!
When people jump off an airplane with parachutes, there are two categories:
Skydiving
This is normally done at lower altitudes, somewhere from 1,000 meters (3,000 ft) - 4,000 meters (13,000 ft). The airplanes used typically for this purpose do not have a pressurized cabin. So when the door opens for paratroopers to jump, pressure outside is almost the same as inside. After the jump, the door is closed and everyone is happy.
This is a Cessna 208 configured for skydiving, with a door visible on the port side of the aircraft.

High-altitude military parachuting
These jumps are done from 4,600 meters (15000 ft) and 11,000 meters (35,000 ft). At 35,000 ft, only 26% of the Oxygen is available when compared to the amount at sea level.
When the door is opened at such high altitudes, amount of available Oxygen is no longer sufficient for humans and everyone needs to wear the Oxygen mask to avoid sickness due to high altitude. More about re-pressurization of cabins is discussed here.
As far as the paratroopers are concerned, they wear special gear, which looks like this:

I guess there's a discrepancy between your answer and ratchet freak's: for a high-altitude jump, would the cabin be depressurized by the opening of the door, or would it be depressurized by other means (valves, etc) beforehand?

@NateEldredge that'd no doubt depend on altitude and aircraft design. I'd guess most aircraft with a pressurised cabin will have means to gradually depressurise, which is far nicer for both the aircraft and its occupants.

The door can't be opened when there is a pressure differential and if it was possible, it would be likely to cause structural damage. The pressure must be relieved by opening the outflow valves before opening the door.

@JanHudec wouldn't that be for a door that opens inward, like on a typical boeing or airbus? for HALO operations, the jump plane is something like a C-130, which has an outward opening door, and doesn't have the same pressure against the door flanges

@rbp: Even for door that open outward (like typical Airbus, by the way) the pressure against the locking mechanism prevents opening them when there is significant pressure differential. And if it did open, they'd end up like UA-811.

I'm no expert on the C-130, but they open in flight no problem

@rbp Most likely because the regulations for high-altitude operations (at least for USAF) state that you must "Ensure zero pressure differential before opening doors". So any time you see them open in flight, they should have already dropped the interior pressure.

that's exactly my point

For low deployment the cabin pressure is already equal to the outside pressure.
For High altitude jumps the pressure is reduced to the outside pressure before opening the door (and everyone puts on oxygen masks).

It does.
When you open the door, Bernoulli's principle guarantees the pressure in the cabin will drop (yes, lower than outside).
The actual pressure difference will depend on various factors like door placement and size, aircraft slip, but mostly and especially on aircraft speed. For a normal skydiving operating speed (say <100kts) this will raise the cabin altitude by no more than 100-200 feet.

OP is asking specifically about paratroopers, not skydivers, so they presumably want to know what happens to a military transport before/after/when the door is opened.

on second reading you are right. Though this basically drops the question to the level of "if I open a door on a submarine will water come rushing in"


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Why is landing on a carrier not handled by a computer?
Landing on a carrier seems like a difficult and risky task for military pilots, especially at night when the carrier should keep low profile and is practically unlit.
It seems exactly like a problem which can better be performed by a computer rather than a human being. A machine can take in account exactly the motion of the carrier and other parameters and land the aircraft precisely where it should. The fact that there should be no enemy aircraft nearby also means that the initiative of a pilot is not needed.
The system can:

Either be exclusively inside the aircraft (doing image recognition of a carrier and trying to determine where to land; not sure how easy and reliable this can get),
Or be presented as a subsystem within the carrier which sends the encrypted information to the authenticated aircraft about the exact position of given points of the carrier and a second subsystem inside the aircraft which relies on this information to make the controlled landing.

Why isn't it handled (yet) by a computer? Are there projects of implementing such system?

If there were projects working on this, they would almost surely be classified, so that part is unlikely to be able to be answered here.

You'll always want a manual override, as with most systems in an airplane (especially military). The only way to make sure people know how to land is by regularly landing manual (one of the reasons of regular qualifications).

@Mast: Why? The military can be quite utilitarian. If the cost of regular manual landing (including human-caused crashes) exceeds the costs of the occasional crash due to a system failure, then the system should be automated.

While it may be computer assisted, it may surprise you that modern aircraft carriers still manage their decks by pushing little toy airplanes made of spare parts around on a picture of the deck known as "The Ouija Board".  Why?  It's flexible and it's reliable.  It means you can still perform air operations with battle damage.

Your premise is couched as an argument from incredulity, and your "especially at night" includes your own uninformed assumptions.  Landing on a carrier seems like a difficult and risky task for military pilots, especially at night when the carrier should keep low profile and is practically unlit.  It's been done successfully for over 80 years.  While hazardous (so what?) the accident rate decreased significantly once the NATOPS methodology entered training and standardization (starting in the 1960's.  Design improvements in later generations of aircraft reduced risk/hazard as well.

The follow up point is to do with my "so what" -- war is the purpose of a navy, war is the purpose of a carrier, and war is the purpose of its air wing.  War is dangerous, as it's about killing and perhaps being killed. I'd have answered that there are already three systems (perhaps  more) already with a computer assist/computer landing system: F-18, F-35C, and X-47B. (But others have covered that to where that answer would be redundant. )  (Why is it not is answered as follows: "well, it is in some cases" and the capability has already been developed ... about two decades ago.

The Navy does have systems that are capable of guiding an airplane in for a landing, and some aircraft can use this system to land completely on the autopilot. However, the system has not always been reliable and still has its limits, and other reasons for not always using this system are similar to those for land-based aircraft.
Further reasons for this technology not being more fully developed yet are probably similar to this question.
A completely automated, unmanned landing on a carrier has been done, but only fairly recently. Although a computerized system is theoretically more capable of performing precise carrier landings, the details are complex. A lot of sensing and control must be worked out. The team behind the X-47B was awarded the Collier Trophy for the advances in aeronautics they achieved.
The sensors and processing that the UAV uses to do this are probably not all standard equipment on naval aircraft. Adding them would create extra expense and weight. If the system requires addition to the carrier as well, there is more added expense. And in a combat situation, do you want to rely on systems that could be jammed or failed? If you are going to land the plane without a pilot, you might as well not have the pilot at all, and this is the route that future unmanned systems are taking, but there is a lot more work to be done before they completely replace human pilots.

Note that there is a gap between a controlled landing (my question) and the lack of pilot at all (linked question). Of course, we absolutely need a pilot when things go wrong, but probably not much when landing in a non-fight, non-disaster situation.

@MainMa That would mean the pilots get not much routine on the kind of landing used in fight/disaster situations, right?

@VolkerSiegel: not necessarily. In Paris subway on partially automated lines where there are still drivers in the cabin, once per day, every driver should drive the train manually without computer assistance to make sure drivers keep the reflexes and are able to control the train in a case of a disaster. The same may work for the pilots. This being said, landing on a carrier might be so difficult that it requires constant, and not occasional practice.

@MainMa: As I understand it, landing on a carrier actually is tricky enough to demand just about as much practice as possible.

Quoting a 1986 report on automated systems is ignoring 20 generations of Moore's Law. That is a factor of one million in processing capability. Pointless, then.

@MSalters But does Moore's law hold true for safety-critical systems as well? Versions of Intel's 80386 were produced up until 2007 for satellite use, more than 20 years after introduction of the version for commercial marketplace. From what I've seen, at least in civilian safety-critical systems, tried and tested beats greatest and latest.

Computers can, in fact, control the approach of a Super Hornet all the way to touchdown.  However, there are limits to the sea state. Gross deviations in glide path resulting from a pitching deck would make a coupled approach impractical.
However, the most compelling reason to hand fly all our approaches is that we fly a $60 million jet. Murphy's Law tells us that the most junior guy, with the least experience behind the boat, is going to get an engine fire, at night, in rough seas.  If our fictional JO wasn't proficient hand flying the ball behind the boat this could result in catastrophic consequences including both the loss of aircrew and aircraft. As often as possible, we prepare for the 1 percentile, not the 99th percentile, so when we do get that engine fire at night we get the upgrade OK, and not the cut pass/ ejection.
Plus, its just plain eerie letting the jet fly itself to the boat, and the current system isn't reliable enough that most guys would feel comfortable letting it fly itself on a regular basis.
Edit:  Heck, new guys aren't even allowed to perform auto approaches (auto throttle) to the boat for the same reason.

Figured you'd chime in here. Are automated landings done beyond the once or twice so a pilot is familiar with the system? Your reasoning for not doing them certainly seems sound.

@FreeMan certain airfields have ALCS systems and aircraft can safely practice coupled approaches.  It also has the added benefit of having lower minimums (the ACLS system as a whole, not just when coupled) than the CV-ILS.  So it gives the aircrew the added benefit of operating in harsher weather.

Reliability.
If the computer or the sensors it needs are damaged (in combat or otherwise) or malfunctioning then the human has to bring the plane in. The only way the human is going to be able to do that extremely precise flying is through regular disciplined practice. They can't practice if the computer is doing the landing for them.
Until you can leave the human behind you may as well make use of them.

I already discussed this point in my comment.

You did and I missed it. Good thinking ;)

Until you can leave the human behind you may as well make use of them. +1 for that, made me LOL!

Then again, if the human is damaged, you do want that computer. And it's a lot easier to carry a spare computer on board than a spare human.

@Snowman: the day a machine has the same incentive as a human to safely land a plane it's flying, despite being itself damaged, is the day the Rise of the Machines begins. Probably also the day it ends.

I flew aboard the USS Nimitz in the mid-1980's in the A7-E corsair. The A7-E was difficult to land because of its turbofan jet engine. You never wanted to find yourself in close, high, with too much power. Your only option at that point, if you haven't been waved off, is to make a big correction by coming back on the throttle. Your correction will be too great, and then you find yourself  low and slow, with no power on the aircraft. The LSO knows all this and so is encouraging you to keep the power on, but then you hit the burble behind the deck and drop out of the sky, most probably when you are coming on with military power. Then there is eternity. The engine takes seconds to spool up as you are sinking lower. The ball has been red for quite a while, and possibly disappeared off the OLS. Now the LSO is not encouraging you anymore, but screaming "Wave off! Wave off!." So better not to be here.
One of my squadron mates was always in the top 8 in grades for landing on the ship. He was good. He taught me 2 things. One, keep power always on the aircraft if it is the A7. Nudge the throttle up. It will want to take you high and so push it down gently with the stick. Fly your approach always just a little fast.
Second, he advised that I use the Link-4A system, or ACLS (Automatic Landing System). This is mentioned in the post by @passel. So the ACLS system was a computer controlled approach. Pilots didn't use it because close in it could do some weird stuff to you, such as make large corrections and scare the flight suit off of you. He used it all the time at night, and confirmed those fears, but he also said you can recover from these sorts of glitches. I think the reluctance of pilots to use it was more about control than anything else. Carrier jet pilots have specific personalities, and the ACLS had a mythology surrounding it.
The other reason it wasn't used was that it was severely degraded in rain because the wavelength of the radar needed to control the aircraft was small enough that it would jump to the rain at times. Not sure what they have today. But like @passel, I was a bit surprised it wasn't mentioned. I swore by it.
There were several issues with taking an ACLS approach. First, if you took it to touchdown you didn't get a grade for your landing, and my goal was always to be in the top 8. But, also there were 2 sorts of ACLS approaches you could request: (1) Mode I all the way to the deck, and (2) Mode II to be dropped from control at 30 seconds to impact, I mean trap. He told me to take the Mode II approach, because it will set you up on glide path and on airspeed, then don't touch anything and you will get an OK 3 wire. If nothing else, make only small corrections and get your OK 3 wire.
I became a dedicated fan of ACLS approaches and marveled at the control inputs as I sat there watching the approach. Kind of like watching a computer play chess. Absolutely amazing. No big corrections and everything done on time and fast. Dropped you at in close all set up with a solid green ball. What was difficult about the approach is that you didn't have the mind-tactile sense of making the control inputs yourself down the pike and it was difficult to stay up with the aircraft. It took a different sort of discipline that had to be practiced. At times it would happen that for some reason or another I might get dropped earlier than 30 seconds, and if my mind wasn't right there I would be playing catch up all the way to the deck. That was often a difficult approach.

There is, in fact, a current Navy project that is aimed at providing automated carrier landing capabilities to aircraft. An overview of this project can be found here and on this PDF (page 24).
It goes by the humorously engineered acronym MAGIC CARPET, which stands for Maritime Augmented Guidance with Integrated Controls for Carrier Approach and Recovery Precision Enabling Technologies.
The goal is to utilize a combination of the flaps, via direct lift control, and the auto-throttle to maintain a constant glide slope.

I'm a little surprised that no one mentioned Link-4A.
I was in the Navy in the late 70's and early 80s, and even then, while I only worked on shipboard Link-11 systems, I heard stories of the related Link-4A system which was used for Aircraft control and communication.
Link-4A had a mode that could auto land aircraft even back then, so it isn't anything new.
From a navy pub at http://firecontrolman.tpub.com/14103/css/14103_66.htm

Automatic Carrier Landing System: The   automatic   carrier   landing   system   selects aircraft in the order of priority from the pattern and enters them into the final approach. During the final approach, a precision radar tracks the aircraft. Correct information pertaining to the approach is transmitted to   the   aircraft’s   autopilot.   When   conditions   are unfavorable  for  a  landing,  the  wave-off  control  is initiated  and  the  aircraft  is  guided  through  a  short pattern and the landing approach is repeated.

But it was also my understanding at that time, the pilots did not care for the system at all. No one would really like the controls taken out of their hands for such a critical portion of the flight.

Computers are excellent at solving problems that are predictable and repeatable. Landing on an aircraft carrier is exactly the opposite of that. Computers can land very accurately on a static runway, but a runway that is moving and bouncing around on waves is an entirely different story, especially when you don't have the goal of broadcasting your position to the world. Keep in mind that we don't even have good self-driving cars, yet. Google probably has the most advanced ones and those are currently capped at 25 mph. Fighters on approach will be doing 6-10 times that speed and moving in three dimensions while attempting an approach to something else that is also moving (somewhat unpredictably) in three dimensions and at speeds likely as fast Google's cars. It's not as easy as it might sound.
While it wouldn't surprise me if systems like this are in the works (it would surprise me more if they were not,) it's unlikely that any such development work would be unclassified. Very few programs designed to add advanced capabilities to military systems are unclassified.

But is a 100,000 ton aircraft carrier really "bouncing around on waves"? It seems that the motion should be somewhat predictable, especially in the last few seconds before landing - the aircraft carrier has so much mass that it's not going to be rising upwards and then a few seconds later, suddenly dropping away without warning (but if it did, the sensors would know and signal a waveoff). Sensors on the aircraft carrier should be able to predict its movement fairly reliably at least for the next few seconds.

@Johnny Carriers are indeed massive, but so are waves. I don't have numbers for the amplitude or predictability of deck oscillations, but I wouldn't expect them to be very small relative to the tolerances needed for a carrier landing. I'd be interested if someone can produce some numbers, though.

@Johnny we are an all weather fighting force and land in sea states so bad that you can see the screws come out of the water at the back of the carrier.  The LSO has to manually control the ball because the IFLOLS is unable to accurately show (more like predict) glidepath.  Also, waves can be bad enough that CAT shots must be timed to avoid shooting aircraft into oncoming waves.

@reirab No experience with carriers, but I have been aboard oil-tankers which are similarly massive. Believe me. They heave in different patterns, but just as badly as a small boat. The power of the sea is something that must be experienced to be believed.

@Tonny Yeah, that's what I would expect. Big boats have more inertia, but the effect on oscillation amplitudes of the arm length between CoG and the edge of the deck shouldn't be underestimated. Even a relatively small change in deck angle can produce a very large displacement at the ends of an 1,100-foot-long deck.

Why is heaving an issue? The question here isn't whether the deck will stay in a fixed position, but whether it will be in a predictable position. The precise reason why a carrier is heaving (would need massive torque to stop it) is also the same reason why the resulting deck position is predictable (lacking that torque, the carrier keeps moving). Just like you'd compensate for forward velocity - you don't want to land where the carrier was a minute ago.

@MSalters But that torque isn't actually lacking. It's receiving massive (and only somewhat predictable) torque from the waves, which is why it's heaving in the first place. Also, again, don't underestimate the effect of that 550 foot arm length on torque.

@reirab: Heaving is an oscillation. A single wave doesn't cause heavy oscillation, and the millionth wave doesn't really alter the oscillation any further. Heaving is so bad because the oscillation is the result of all those waves combined. As for the "550 foot arm length", the ship's length in fact a good thing: it means that the ships' moment of inertia is massive. The arm length needed to calculate torque exerted by waves isn't the ships length, but half the wavelength. And that's generally a lot less than 550 feet.

There are perfectly functional self-driving cars.  The only problem is to blend in the traffic.  Flying a plane over the open sea is easier than driving a car in town.

There are three main reasons: reaction time, sensor inadequacy, and predictive intelligence. Taking them in order...
For certain types of stimulus-response activities a human can respond faster and has a better reaction time than a computer. For example, imagine it takes 50 milliseconds to sense something completely, 100 milliseconds to transmit that information to the computer, and 100 milliseconds for the computer to figure out the right action, and 80 milliseconds to transmit the response to an actuator. That's a total of 330 milliseconds. A human can respond to an event in 200 milliseconds. Note that for some types of actions the computer's sense-and-respond loop could be much slower than the numbers I have given. For example, most artificial intelligence algorithms are complicated and take a lot longer than 100 milliseconds to complete. Some types of optimization algorithms, like dynamic programming have unpredictable running time, so they might finish in say 50 milliseconds with one set of inputs and 1000 milliseconds in another.
The next big problem is that the human eye, which is wired in parallel directly to the brain, is a far better sensor than anything available to an aircraft. It has a wider field of view, higher bandwidth and better focusing and movement ability than any man-made optical system. Currently the sensors on planes are things like inertial measurement units, GPS and flight instruments (altimeter etc). These sensors are not enough to reliably land a plane on flight deck except in very calm conditions.
Finally, landing on a moving flight deck requires predictive intelligence. You basically have to guess where the deck is going to be in the future, combine that with estimated wind gusts, and then calculate in the future motion of the aircraft given a range of factors. Putting all this together is currently way beyond the ability of artificial intelligence systems.
There are actually a lot of things MUCH simpler than landing on a flight deck that we cannot do. For example, if the Air Force could just land UAVs in heavy cross winds that would be a huge win for the military. Currently, if the cross wind component is above a certain amount we cannot even land UAVs because they would crash. This is because the UAV cannot react fast enough to a wind gust the way a real pilot can. We lose hundreds of millions of dollars in UAVs every year because of simple stuff like this, and remember these are manned system that are only semi-automated. Fully automated aircraft don't even exist, because they would crash so much as to be much more costly and dangerous than useful.

There isn't a whole lot of guess work conducted in the carrier landing pattern.  It is a very mechanical, rehearsed procedure.  That's not to say that the entire approach is linear, there are some aspects that require feel and experience (such as the feel of the jet in relation to the movement of the ball), and the best ball fliers generally approach ball flying as an art, but you can get by with excellent mechanics.  Also, the ACL Mode 1 approach is a coupled carrier approach all the way to touchdown, and has been around for awhile.  Its rarely used though.

@SHAF How does this relate to my answer? Are you suggesting it does not require predictive intelligence to do a carrier landing? Are you a naval aviator?

Can you provide some evidence that a human can react in 20 milliseconds?  I can't find anything that says human reaction time isn't an order of magnitude worse than that.

@jbarker2160 Sorry, brain fart, my numbers were off by a factor of 10, I have updated the post.

@TylerDurden Your answer overemphasizes the predictive intelligence required.  Nobody is guessing their way onto the flight deck.  There is a whole slew of information available to the pilot, be it the ball, the carrier itself, the LSO, the GCA controller, the TACAN, the CV-ILS or the ACLS.  All glide slope systems on the boat (like the ball) are calibrated to operate correctly with a moving deck.  Furthermore your conclusion that its impossible is incorrect because ACL Mode 1 is an automated approach that's been around for a very long time.  The Navy has also landed drones on the boat.

100ms, 80ms is about the right latency to send data to the other side of the planet. Sending information to a local computer wouldn't have that kind of latency.

Your numbers appear to be completely made up, here's a differe - an off the shelf sensor can easily provide 300Hz update rates, so if you need to look at 5 samples to see a trend, that's around 16msec and with a 115kbs serial interface, that's 1msec to transmit the data to the computer (lets pad it to 10msec). 30 msec gives the 3Ghz CPU about 100M clock ticks to decide what to do, another 10 sec to send to the actuator and you're up to around 65msec.

@SHAF I work on intelligent systems for the military and last year I worked under contract for the Air Force with Emergent Space Technologies that did all the differential GPS localization science for the drone landing demonstrations you are mention, so I think I know what I am talking about.

@TylerDurden I'm a carrier qualified Naval Aviator. I've done what you're talking about.

@Johnny Look, you think its so easy making robotic DO IT, the DOD will pay you millions. I actually work for a company that makes fully autonomous military robots and UAVs. Reaction times are one of the primary serious and technology-limiting problem.

@SHAF Ok, maybe you as a human aviator find prediction easy and find it trivial to predict the effects of vortex turbulence and deck motion, but if you read any of the dozens of papers on the subject, you will find programming ACL systems involves very complicated and difficult prediction problems which are currently unsolved.

@TylerDurden I agree with much of what you said, I just believe you over emphasize the predictive intelligence required, specifically that you equate it to guess work. The ACL equation is most certainly complicated, but it is no where in the realm of impossible as the Navy has demonstrated. The real issue is that the Navy likely doesn't care enough to invest the money required to have a super accurate system since we'll likely continue to hand fly it anyway. The only real usefulness I can see for such a system is for unmanned carrier operations.

I find it hard to believe that a WWII-era fire control computer could shoot an airplane out of the sky or target a moving ship 10 miles away, yet modern-day computers and sensors aren't enough to land an airplane on a ship.

@TylerDurden - I never said it's easy, just that it's possible. Perhaps not worth it for manned planes since the pilot still has to fly a significant number of manual approaches to stay in practice just in case the automatic system is down, but it seems that the physics are well known and the appropriate sensors and computer power exist. And, this will be a solved problem before long to enable unmanned drones to land, even the US Military can't afford to drop $40M+ drones in the ocean when they can't be recovered

Worth noting that avionics generally are miles behind the cutting edge technology which may be the cause of disparity in the numbers here.  Cutting edge FPGAs can receive, process and transmit data in latencies measured in nanoseconds. See also http://space.stackexchange.com/questions/247/why-are-spacecraft-data-systems-obsolete-at-launch.

100 milliseconds for the computer to figure out the right action well, if it is hamster-powered, maybe, on my PowerPCs, it takes 1ms in the worst case, 80% of the time it stays below 0.7


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Why do passenger jets accept input that will cause the aircraft to perform dangerous maneuvers it was not designed for?
Examples:

A bank angle > 45 degrees is considered an “upset,” putting the plane in a position that can lead to a loss of control.
A pitch > 20 degrees can possibly be dangerous and cause the aircraft to stall (depends on many factors or course).

These are just a few examples of limits that most passenger jets have. Now why is it that the plane actually accepts input that will cause the aircraft to go beyond these limits? What possibly use could “diving” straight towards the ground or towards the sky have? Or having a high bank angle that will almost certainly cause stalling and loss of control?

Not all passenger jets permit pilot control inputs to cause the aircraft to enter a dangerous attitude. Most Airbus aircraft, operating in "normal law",have flight-envelope protections intended to prevent this. Some Boeing aircraft have flight envelope protection but this can be defeated by using "excessive force".

Automatic "flight envelope protection" is discussed in this wikipedia article and provide examples of situations that may justify manual control. A well known case is China Airlines Flight 006.

Not an answer but related.  Modern fighters enable the pilot to make maximum control inputs.  The computers will figure out what control surface deflection is applied without breaking the pilot or the airplane and fly on the edge of the envelope.  Early F16 development made a lot of breakthroughs in this area and one of the problems was that the aircraft could make maximum input maneuveurs without breaking the airplane but the pilots could not withstand the G. The computer algorithms were tweaked to protect the "wetware" a little more.

@mins: China Airlines flight 006 is a great example of why flight envelope protection is superior. If the plane had flight envelope protection, it would have never entered the spiral dive in the first place! (And the flight envelope protection systems are smart enough to switch to direct control if the flight envelope is already left e.g. due to really extreme turbulence).

@JanHudec. Pitch-direct may not be available after abnormal attitude detection. It seems this would be Pitch-alternate with load factor protection still active, preventing a pullup at 5.5g. Not sure however, correct me if I'm wrong.

@mins: I think yes, it would be alternate. still with g-based command. After all if turbulence upsets you you still don't want to break the aircraft apart by pulling too hard. If it won't let you in a spiral dive (and it won't; the system maintains level to 33° bank and will automatically reduce higher bank), you won't need it.

I believe that my Ford Focus will let me input commands (steering, braking, accelerator) that would cause it to perform dangerous maneuvers it was not designed for. And the Ford Focus is designed to be piloted by general (licensed) consumers, not by rigorous highly-trained professionals as is a Boeing 787.

@dotancohen Not only is your Ford Focus not carrying 300 passengers and 300,000 liters of fuel (or maybe you tricked it out some?)  I wouldn't use your Ford Focus' lack of safety features as a positive.  Driving is so much riskier than flying commercially.

@JanHudec The Wikipedia article on China Airlines 006 mentions the attempt to restart an engine at too high an altitude, and failure to apply rudder. I can imagine it might have helped if the autopilot of a B747 had rudder control, but what about the altitude? Should the avionics force the aircraft to descend to 30000 feet when an engine flames out? (That's not a rhetorical question, by the way.)

@DavidK: No, it shouldn't. It should just prevent it from entering a spiral dive because of it. Which is exactly what the Airbus one would do. It does not automatically dial in the rudder trim, but it would apply aileron and elevator which would make the aircraft fly with more drag (in a slip), in a circle because the aileron would only be applied as reaction to too much bank, but it would not be loosing altitude. That would give the pilots time to regain situational awareness and fix the problem.

@JanHudec If I read the article correctly, the autopilot was already applying the maximum aileron control (I don't know about elevator), but it wasn't enough. I'm not sure about this exact incident, but I believe an aircraft with four engines can safely maintain a higher altitude than the same aircraft with three engines running, so if one engine flames out near the maximum sustainable altitude the pilot must descend. If the pilot does not, the laws of physics will accomplish the same goal, but not in a nice way.

@DavidK: Well, no, it wasn't. The autopilot has a limit force and disconnects if it would need to apply greater force. And I think the aileron returns to neutral when that happens and pilot does not take over. In Airbus the flight envelope protection is a separate layer that will not disconnect unless the necessary sensors fail or pilot turns it off manually and that will apply as much control force as needed. I have read reports about testing engine failure in Airbus and the described behaviour was: banked somewhat, entered slipping turn, maintained vertical speed.

@JanHudec What I see on Wikipedia is: "As the speed decreased even further, the plane began to roll to the right, even though the autopilot was maintaining the maximum left roll limit." I did not understand that to be "disconnected". But neither do I believe everything I read on Wikipedia. On the other hand it would not surprise me if the autopilot had one rule for what to do when the pilot dials in an excessive maneuver somehow on the control panel, and a different rule for what to do when it is merely trying to maintain the aircraft's existing attitude.

@DavidK: I don't have detailed knowledge of that system, but I don't believe the ailerons would not have enough authority if applied manually; the yaw-roll coupling is not that strong. It would be the autopilot limit that was exceeded. And then the autopilot disconnected. They all disconnect when they can't maintain the set parameters, because it normally means something fails and the autopilot is not able to fix it. And then the plane was completely uncontrolled.

Generally speaking, pilots don't like it when a computer interprets or limits their actions. They want final control. They don't always get their way on this but that's their preference.
If I recall correctly, Boeing tends to stick with the philosophy that "the pilot is the final arbiter." Airbus is more likely to preempt pilot inputs and modify them.
Although the majority of crashes and incidents wind up being pilot error, there is a serious flaw to modifying pilot inputs. That flaw is in the case of systems failure.
By definition, failure modes involve things going wrong.  When things go wrong it's effectively impossible to plan, in automation, for all contingencies. People are much better at responding to the unknown than automation systems are.
Take for instance the rule that "bank angles > 45 degrees are dangerous, and are therefore prohibited". How does the plane know that the bank angle is > 45 degrees? Well it's a sensor of course, but what if the sensor has failed? A failed sensor will either signal to take action when none is needed or fail to signal when action is called for. What if the control surfaces have failed and the plane cannot correct the bank angle?
The usual answer to that is redundant systems, high reliability parts and design, etc.  All those are great of course and certainly help a lot. However we still have incidents and accidents.
In the end the question is: Who do you trust more? A pilot or a machine? And statistics and science only help you part-way here. A person's experience, biases and feelings will have a lot to say about how they answer. And by "person", understand that I'm including the customers, the paying public.

+1 For the Boeing vs. Airbus comparison. In Boeing jets, the pilot is the final authority in flying the airplane. In an Airbus, he is just a voting member.

+1 especially for sensor failure comment. It's more likely a plane will need the pilot to do something irregular and unexpected because of situations including malfunctions and things the computer may not be able to even have good data on. When is the pilot going to risk a dangerous maneuver without a a good reason?

Not even a fully working sensor can tell you dependably what roll angle the aircraft is flying. And, btw, newer Boeing planes (777, 787) restrict the pilot's actions quite as much as Airbusses do since the A320.

@PeterKämpf Heck, it's hard enough to tell how fast you're going, let alone the angle.

Re "what if the sensor has failed?", it was of course a failed sensor which caused AF 447 to switch to 'alternate law' - switching off the limits that would have prevented the pilots from stalling the aircraft: "once the computer lost its airspeed data, it disconnected the autopilot and switched from normal law to "alternate law," a regime with far fewer restrictions on what a pilot can do. In alternate law, pilots can stall an airplane."

This (highly upvoted) answer consists of opinion. Pilots who have flown both Boeing and Airbus have more nuanced opinions (example 737 & A320 pilot). I'd like to see some more objective/factual references for the assertions made.

@MichaelKjörling: Indeed, from that very article: "A second consequence of the reconfiguration into alternate law was that "stall protection" no longer operated. Whereas in normal law, the airplane's flight management computers would have acted to prevent such a high angle of attack; in alternate law this did not happen. (Indeed, the switch into alternate law occurred precisely because the computers, denied reliable speed data, were no longer able to provide such protection – nor many of the other functions expected of normal law)."

Should also consider the human pilot's track record -- in the US alone, there's an average of 87,000 commercial flights per day. There's been a few commercial airline accidents in the news as-of late, but typically flying in an airplane is significantly safer than most of your daily activities. Statistically you are more likely to die choking on your lunch than in an airplane crash. It should speak to how well a trained and experienced airline pilot can fly on average; heck, even the poor airline pilots are incredibly safe statistically speaking.

The argument of "pilot having full authority" is somewhat vague and I think unjustified. The Airbus normal law is designed to allow full certified flight envelope. For example the max bank angle of 67 degrees will give 2,5Gs in level flight - which is the structural limitation. In sudden distress it is safe to make maximum side stick input and you will get maximum available performance.
The safety comparison (by Airbus) presents that fly-by-wire controls have reduced number of loss-of-control incidents roughly by factor of ten.
https://accidentstats.airbus.com/statistics/generations-of-jet

The philosophy is that the pilot knows best. If they need to make a maneuver, they should be trusted to do so.
Although there are absolute limits such as structure, other limits are less exact and depend on conditions (and even structure is built to withstand additional margins, failure, and damage). Something that qualifies as an "upset" is certainly not routine, but it is not necessarily fatal either, and can be used to deal with certain situations.
A big reason for abrupt maneuvers would be avoiding an obstacle. In the case of terrain, this would generally be a steep climb, or possibly a steep turn. But this could also be another aircraft, in which case the pilot may want to descend quickly.

Indeed. There have been fly-by-wire that would refuse "dangerous" input. They had to go back in and convince the control software to permit the maneuver if the pilot moved his controls to the stops after a couple of crashes that might have been prevented by an out-of-range maneuver.

@Joshua: Which ones? For Airbus many cases are known where the protections prevented crash, a couple where they failed to prevent it (usually when the sensors failed so it couldn't provide the protections) and no cases where the outcome without them would have been better.

I'd be hard pressed to find it again, sorry. It was taught as standard material in CS courses 10 years ago. The problem is the flight computers can't see ahead and so will fly into a mountain rather than attempt too radical a maneuver.

@Joshua Huh? Why exactly would a flight computer be unable to "see ahead"? While there might be a few cases where the computer caused a crash, the statistics are very clear that the vast majority of crashes are caused by human error. People just don't like the idea of not being (or at least having a fellow human) in control, even if it'd objectively safer.

I did find a reference in this book to Iberia 1456, where the protections incorrectly pitched the aircraft down, causing it to land very hard.

Well that's not the one but it's the same essence. The computer doesn't know the current trajectory will crash and even in stall airspeed will be traded for altitude. The one I recall the plane collided with a mountain that should have been cleared if the pilot's steering was followed.

And a related aspect of this--in an emergency the correct maneuver may be to risk the airframe.  When you see the mountain in front of you an overstressed airframe is the least of your worries.

@LorenPechtel I'd say it's the greatest of your worries, actually. Collisions tend to be quite stressful.

Let's just focus on roll. The same command that can be used to roll the aircraft from 0° roll angle to 30° can be used to roll it from 30° to 60°. Who is to decide at what roll angle the airplane is and that from now on no further roll commands are acceptable?
A computer-controlled FCS, obviously, if we decide the pilots cannot be trusted. But can we trust the FCS more? What would be the basis for it to establish the correct roll angle?

Gyros? They need to be calibrated once in a while, because all gyros drift. Some more, some less, but no technology can prevent them from showing dangerously wrong readings when they are left running long enough.

Accelerometers which show the gravity vector? As soon as the airplane flies a coordinated turn, it should be obvious that they point only away from the lift vector. No dice.

Radar altimeter at the wingtips? Fly high enough and they become useless. This might work for low-level flight, but not in all flight phases.

Camera and image processing to find the attitude towards the horizon? Stops working at night or in fog.

I could extend the list, but by now it should become clear that this is not as easy as it sounds. Especially the FCS design for autonomous UAVs is quite tricky and needs to correlate the inputs of different sensors in order to establish level flight. This was learned the hard way by Aurora Flight Sciences when flight-testing their Perseus A prototype. Relying on the gyro alone, the team did not realize that the sensor drifted away and commanded increasingly steep bank angles. When the aircraft disintegrated, the team did not even immediately realize what had happened because the maximum sink rate value on the flight data downlink corresponded to just 20 m/s - it just got stuck at -1023 counts. The aircraft prototype was totally destroyed in the accident.
 Perseus A before its final 21st flight.
I guess this is the last forum on the whole Internet where it needs to be explained that relying on perfectly functioning software is foolish. Somehow, human pilots are still better at resolving unforeseen difficulties, for the same reasons why they sometimes screw up in inexplainable ways.

So the Perseus A suffered its own form of the spatial disorientation that afflicted the human pilots of CA006, AF447 etc?

Well, that's one team of engineers who will remember that gyros drift. There's no substitute for experience.

If turning at constant speed, then the aircraft is also constantly accelerating so indeed, not an easy problem to solve.

Pilots also loses their sense of level flight when in fogs without their instruments, and instruments can lie, as seen in this accident, and pilots have gotten into accidents when they trust their artificial horizons too much. It's the Perseus software's fault that it relies too much on gyro, but you only need to teach a computer once. Human has a natural affinity for visual flight and adapting to unforeseen circumstances, but computers have better potential at instrument flying and avoiding accidents in the first place. Both need to be properly trained/written to be effective at what they do.

+1 for "relying on perfectly functioning software is foolish". To quote a renowned computer scientist, E. W. Dijkstra, "[software] testing can only prove the presence of bugs, not their absence".

But... can pilots really perform better at identifying the roll angle in difficult conditions when compared to computers?

@JonathanReez: As others have pointed out, properly written software has an edge in some conditions. On the other hand, Perseus A crashed on a clear day in broad daylight, and even an inexperienced pilot would had performed better than this particular autopilot.

Also, how would the radar altimeters know whether they're pointing straight down, or, instead, at an angle?

Most new designs do not accept such inputs. That includes:

Airbus models from A320 onwards (includes A318 and A319 that are variants of A320).
Boeing models B777 and B787.
Sukhoi SuperJet Su100.

Airbus has roll limit 65°, not 45°, but it automatically returns to at most 33° without constant pressure on the stick. I can't find explicit pitch limit, but it has alpha-limit (angle-of-attack, depends on type, 17° for A320, pitches down not to exceed it), maximum speed and Mach limit (pitches up if exceeded) and minimum and maximum wing loading (vertical acceleration, -1G to +2.5G clean, 0G to +2G with flaps)

But the direct law is enabled in many situations when the computer sees a problem and will accept any input.

@VladimirF: Depends. On Airbus it will degrade to alternate law where the stick deflection still correspond to vertical acceleration and roll rate (so they are still limited) unless the problem is with inertial reference. Boeing always interprets yoke deflection as control surface deflection, so it does not have alternate law. As for Su100, it aims for Airbus commonality, but it might differ in this; I don't know.

Airbus degrades to Alternate law (/w or w/out protections) after single or some dual failures when the computers can still figure out how to protect the aircraft.  Some dual and most triple failure degrade to direct law. It's not about the control surface deflection (control law) but about computers having enough info to enact a protection

The pilot must be trusted over anything else (and even Airbus planes accept any input in Direct law, Normal law doesn't always apply). Any unforeseen situation may arise during the flight. Computers cannot handle every abnormal situation.
A good example of such a situation is the case of FedEx Flight 705. The pilots were attacked with a sledgehammer by a hijacker. They would probably be dead, if not for the extreme maneuvers attempted. They pushed their DC-10 plane far beyond its limits (bank angle up to 140°, overspeed near mach 1.0). Had the computer prevented them to do this, the plane may have crashed and they'd be all dead.

As an airline pilot (and test pilot) I like to keep my airplane under control (like all other airline pilots do). There is Boeing school and the Airbus school. Boeing airplanes will warn you not to get into those envelopes. Airbus won't allow you to get in that regime (envelope protection). In any case you can override that by changing law or disconnecting flight computers. In extreme cases where the situation is desesperate, I won't hesitate to overrule the computer and go into the extreme envelopes if I need to save lives. Bear in mind that everything in aviation is designed with a safety factor of 30% to 60% in some cases.
So to answer your question: You are at FL370, sipping coffee, you have a fire on board. The aircraft limits your vertical speed and speed during descent (or bank angle if you want to turn back). Would you be OK with such limits? Myself, no. We are paid big bucks in the front to make decisions.
Second scenario (fake). FL340, cruise, your TCAS failed but you don't know (like I said it is fake scenario). Suddenly you see the other guy heading towards you. Same FL. But your computer says "sorry you cannot pull hard because of the G protection", you hit the other guy: you are legally dead because you have not stressed the airframe.
Last example: Just after departing for a 16-hour flight you have fire in cargo. The fire extinguishing systems are unsuccessful. You must land, but you cannot because you are overweight (by 100 tons I would say at least). What would you do?
Hope I've triggered something about your question. BTW I'm on 777.

A couple of reasons, but essentially it comes down to "Because sometimes it may be the lesser of two evils"

Instruments go wrong. Pitot tubes break, gyroscopes go wonky etc... Sometimes, the pilot really does know best. Autopilots disengage when they aren't sure what to do, the same applies to autopilots limiting the control in a fly by wire setup... what happens when the aircraft thinks it's stalling but it isn't? It tries to prevent the nose being raised, or actually lowers the nose, until the aircraft hits the ground.
A "virtually guaranteed stall" may one day be a better option than an "actually  guaranteed collision" - if your choices are pull up hard or fly into a mountain, I'll take my chances with attempting to recover from a stall.

1 is unlikely (and considering how often aircraft fly under IFR, would likely affect the pilot the same as the autopilot), 2 is probably even more unlikely, but is the situation which would prompt thousands of "Why can't the pilot over-ride the autopilot?" questions.
At the end of the day, people still mistrust computers. They may get it right 99.9999% of the time, but they still can't "think on their feet" like a human.
Now, there are elements of this already in aircraft design. For example all modern airliners have audible/visual warnings when dangerous situations are presented (high sink rates, stall warnings etc). And Airbus goes further, using "fly by wire" controls which do indeed prevent most "normal" stall situations in what's called "Normal law". Airbus does, however, give final control to the pilot if the computer is not 100% sure of the situation.

Another thing to consider is that knowledge required for controlling the aircraft is increasing with such systems in place.
There nearly was a plane crash in Germany some years ago (see this video showing the landing), when a plane was landing during strong side winds. The problem was that the flight controls reacted differently when the plane touched ground or not. That was not known by the pilots. They managed the situation, but it could have been easier for them with knowing that behavior. This behavior even was not documented in the manual.
EDIT:
From the investigation report (thanks to @DeltaLima):
Section 3.1

The pilots could not have been aware of the specific flight system control response characteristics during a landing with a gusty crosswind und were, therefore, unable to incorporate it into their decision taking process.

And later on in this section

• When the left main landing gear first touched the runway, the lateral control system condition thus met all the requirements for the transition from Flight Mode to Ground Mode, so, the system switched from lateral Flight Mode to lateral Ground Mode even though the aircraft was once again in the air.
• The aircraft was designed so that the effect of lateral controls (along the longitudinal axis) would reduce by about one half of full deflection as soon as one main landing gear touched down.
• The reduced effect of controls was not documented in the system description and was unknown to pilots or the training department.
• During the landing, the aircraft's system behaviour contributed to a flight attitude which was unintended and undesired by the pilots and ground contact with the wingtip could not be prevented
anymore.

Hello SpaceTrucker, welcome to Aviation.SE. I think you are referring to this accident, investigation report here. Feel free to include it in your post.

@DeltaLima Thanks, i had that accident in mind.

One of the problems with commercial aircraft is the sheer number of hours of flight that any given aircraft type will see and the vast number of different conditions and failures that will inevitably arise during all that flying.
I had the job of writing the software that validated the control laws for the center of gravity management systems of several of these beasts. There's really no way to completely analyse all possible 'modes of flight'. The FCGMS system wasn't quite in the same category with the flight controls, but it WAS a 'safety of flight critical system'. There was a lot of validating involved, and this was just purely functional test, not all the vast array of unit level testing that went into the flight software. The thing is you have 20 boxes on these planes, all doing critical stuff, all built by different people, etc.
In the end someone has to be able to take hold of the stick and PULL UP! when its required and get a direct response. Its quite true that this capability might be as often detrimental, but you just cannot and never will be able to, completely analyse code and know what it will do when pieces are falling off the airplane.

Let's think of this from a different angle. Sometimes it's not just pilot input that would put the plane into an unsafe attitude. Now if the plane is forced into a heavy bank, climb, or dive through external forces but the pilot is limited in what response he can give to the plane to adjust....
Also think of high fog and communication issues. and two planes are in risk of collision. The plane says to the pilot "nope you can't bank because that would put us at risk".
Under normal flying conditions, you'd want to fly within operating parameters. But when the proverbial stuff hits the fan, you want full control to attempt to avoid a tragedy.

Your question can be answered by a simple reductio ad absurdum argument. If the airplane knew what inputs were safe, then why have any pilot at all?
In fact modern commercial flights already pretty much fly automatically, and they do have mechanisms to avoid potentially dangerous inputs. The problem is that failure detection is hard because when something has failed, the system (by the definition of failure) does not have valid data to make decisions. It is the same reason crazy people may not know they are crazy. The pilot is there to make decisions in the face of ambiguity.
Of course sometimes the pilot makes the wrong decision too, and the airplane crashes. See Air France 447.
But more often they make the right decision, and everyone lives. See the Gimli Glider.

Generally limiters are optimized and designed to fulfill a specific function or role relation such as the Airbus full stick aft to achieve max AoA during terrain avoidance. However, not all limiters are foolproof, despite what is advertised by the manufacturers. Limits can be exceeded depending on how the limiter functions and FCS are implemented eg using dynamic manoeuvres such as repeated pushing and pulling at reasonant frequency or sudden manoeuvres at speeds for which the FCS wasn't designed for or in degraded aircraft modes. Although these possibilities exist, they are usually not relevant because these are corner cases unlikely to be encountered.

About the AF accident. The basic of flight: PITCH, POWER, PERFORMANCE. Meaning that when something is wrong you disconnect all automation and get manual control of the ship. The key beyond the PPP is that for almost any given situation applying a pitch, a power setting and a configuration (flaps, slats, gear....) you will kee you flying. With automation those things are long gone. During a flight, each hour I was writting down power setting, wind direction, temperature (among fuel and other legal stuff). Those whom started flying without autothrust and on turbo props know what it s like.

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Pilots want to have control over the aircraft. Personally, if I was an ATP I would never fly an automated FBW aircraft like an Airbus for that exact reason.
You don't to be in a spin or death spiral and then find out you can't get out of it because "the computer will not allow that input", and trust me if you are in an aircraft that goes into a spin you most definitely want the pilot to have FULL control over the aircraft.
You may think: if the computer is controlling the aircraft it will never enter a spin. Not so. Some thunderstorms can arise very quickly and unexpectedly and are so violent they can force the aircraft into a spin or other attitude which the computer has zero chance of reacting to correctly. "Good" computers turn themselves off at that point.
The most advanced FBW aircraft have a feature called "automated recovery" which allows them to automatically recover from some stall conditions, but in extreme conditions the changes in attitude will be happening so fast that you need a human to do it.

authors: Vladimir F Героям слава